1. Field of the Invention
This invention relates to an improvement of a hydrostatic pressure linear guide device which is adapted as a work-table or a tool-table of a grinding machine. More particularly, it relates to a hydrostatic pressure linear guide device which comprises a slide unit having a static pressure guide surface to which plural number of pressure pockets are formed which cooperates with a regulator controlling fluid flow from a pump to the hydrostatic pockets, and a guide unit having a pair of guide rails.
2. Description of the Prior Art
Hydrostatic pressure linear guide devices are used as a work-table or a tool-table of work-machining apparatus such as a cylindrical grinding machine, a cam grinding machine, a crank shaft grinding machine, a dicer, NC lathe, and so on.
The hydrostatic pressure linear guide device includes a guide unit (guideway) having a pair of right and left rails formed on an upper surface of a bed, and a slide unit (slideway) having plural oil static pressure pockets formed respectively on a lower surface and a side wall of the slide unit. The device also includes one or more controllers controlling oil flow from an oil pump to the oil pockets in which a static pressure between the rails and slide unit is maintained at a predetermined level. The slide unit is driven by a linear motor, a servo-motor with a ball-screw or hydraulic cylinder.
Conventional hydrostatic pressure linear guide devices have guiding faces provided with absolute flatness wherein the sliding table is floating on a static oil membrane to obtain an ultra precision straightness. The guiding faces of the slide unit are not in contact with the pair of right and left rails of the guide unit during sliding.
Hydrostatic pressure linear guide devices are known in the art as described in JP-A-63-74536, JP-A-6-735, JP-A-7-299684, and JP-A-2000-74065.
The JP-A-63-74536 reference shows a static pressure linear guide device including a guide unit 1 having a base 201 and a pair of rails 2 on the upper surfaces of a pair of wear plates 20a, 20b having slant faces 60 supported by the base 201. The device also includes a slide unit 3 having a slide base 3a and a pair of wear plates 3b, 3c whose upper surfaces are positioned facing the guide surfaces of the wear plates 20a, 20b. The device further includes a pressure feeding means which is provided for mutually independently regulating pressures respectively fed to upper, lower, left, and right pockets arranged to the slide unit 3 to form static pressure bearing. A return flow of the pressure oil fed to a static pressure generating unit is controlled by a static pressure squeezing land section A2 and the static pressure is generated in the static pressure generating band width L, and this static pressure is applied to the guiding face 21 of the rails 2 and the guided face 31 of the moving slide unit 3 to keep them at a fixed distance. A land section B1 holds a fixed small gap E so as not to bring the guided face 31 and the guiding face 21 into contact by the action of the static pressure.
Accordingly, when the slide unit 3 is moved in a straight line direction against the rails 2, a fluid between many recesses 6 having slant faces 60 and opposing faces to them is compressed, and the dynamic pressure is generated.
The JP-A-7-299684 reference shows a static pressure linear guide device composed of a guide unit 1 having a pair of right and left guide parts 3, 4 and a slide unit 2, where the slide unit 2 has a pair of right and left slide recess parts 5, 6 provided with both sides of the slide unit brought into contact with the upper face parts 3a, 4a of the right and left guide parts 3, 4, oil pockets 7, 8 in the upper face 5a, 6a of the slide recess parts 5, 6 oil pockets 7, 8 in the under face 5c, 6c of the slide recess parts 5, 6. The slide unit 2 has oil pockets 9, 10 at the end face 5b, 6b of the slide recess parts 5, 6 and has the first and the second shut-off valves 25, 26 closed to stop the supply of pressure oil into oil pockets 7, 8, 11, 12 for lowering the slide 2, and the upper face parts 5a, 6a of right and left slide recess part 5, 6 are thereby brought into contact with the upper face parts 3a, 4a of right and left guide parts 3, 4 ,the third shut-off valve 27 closed to stop the supply of pressure oil into an oil pocket 9 for moving a slide 2 to the left, and the fourth shut-off valve 28 closed to stop the supply of the pressure oil into an oil pocket 10.
The JP-A-2000-74065 reference shows a static pressure linear guide device including a guide unit 10 having a pair of right and left guide parts 11, 12 formed on an upper surface of a bed 10 and a slide unit 20 sliding along to the pair rails. The rails 11, 12 are furnished with rectangular cross-sections and formed so as to be overhung from the upper surface and a side surface of the bed 10 along the both side surfaces of the bed 10. Step parts 17, 18 are formed on the both side surfaces of the bed 10 so as to face to lower surfaces of the rails 11, 12. Wear plate parts 23, 24 are installed on lower end surfaces of leg parts 25, 26 of the table 20 and are guided by the lower surfaces of the rails 11, 12 on their upper surfaces and by upper surfaces of the step parts 17, 18 on their lower surfaces respectively. Static pressure pockets 31–38 are respectively formed on a lower surface of the table 20, the upper surfaces of the wear plates 23, 24, the lower surfaces of wear plates 23, 24 and side walls of the groove parts 21, 22 in which the rails 11, 12 are stored.
A load of the slide unit during sliding concentrates on the joint bolts of the guiding surfaces of the wear plates of the guide unit in those hydrostatic pressure linear guide devices, so that the wear plates are bending when massive work or tool is loaded on the sliding table. Consequently, the straightness of the obtained machining-work is about 1.0–2.0 μm per 1000 mm.
The flow rate Q is proportional to the third power of the gap height or the height h of the pocket gap through which the fluid flows. The result in the case of laminar flow is the condition Q˜p/h3/η, where η denotes the dynamic viscosity of the fluid and p is a pressure. The smaller the gap and the higher the hydrostatic fluid pressure is, the greater the stiffness of the hydrostatic pressure guide device is, but the consumer demand of fluid for raising the hydrostatic fluid pressure is increasing.
A floating distance (gap) between the table and a grinding head of the conventional hydrostatic pressure linear guide device depends on the feeding speed of the slide unit. In the conventional hydrostatic pressure linear guide device, it is variable in a range of 0.01–0.50 μm when the feeding speed is 1–25 m/minute and the shock of the reverse of tool-table is diminished in about 2.4–2.8 seconds.
The JP-A-6-735 reference shows a static pressure linear guide device including a guide unit 1 having upper, lower, left, and right guide surfaces 2a–2d, and a slide unit 3 having slide surfaces 4a–4d positioned facing the guide surfaces 2a–2d that are guided and supported to upper, lower, left, and right guide surfaces 2a–2d. A micro uneven surfaces 10 is formed in either the guide surfaces 2a–2d of the guide unit 1 or the slide surfaces 4a–4d of the slide unit 3, and a pressure feeding means 9 which is provided for mutually independently regulating pressures respectively fed to upper, lower, left, and right pockets 5a–5d arranged to the slide unit 3 to form a static pressure bearing.
In the hydrostatic pressure linear guide device, load of the slide unit concentrates on a frame of the guide unit during sliding, so that the stiffness of the hydrostatic pressure linear guide device is greater than those of the hydrostatic pressure linear guide devices described in above-mentioned three patents, but it is necessary to use greater power moving the slide unit.
The stiffness of the hydrostatic pressure linear guide device, the motor power moving the slide unit and utility of fluid depend on cross-sectional shapes of the slide unit and the guide unit and the position of the hydrostatic pressure pockets on the slide surface. The hydrostatic pressure linear guide devices described in the aforesaid patents may not fit for a large slide table having a length of more than 3000 millimeter or to be loaded with a massive tool.
When the dimensions of work-pieces to be machined become greater, the land length and width of a slide unit also become greater to increase the weight of the slide unit. Furthermore, tools also grow in size to increase the weight of the tools on the slide unit as well. In comparison with oil static pressure linear guide devices described in JP-A-63-74536 and JP-A-7-299684, an oil static pressure linear guide device described in JP-A-2000-74065 includes a slide unit having more sliding surfaces and higher stiffness in the vertical direction in machining. Therefore, it is suitable as a hydrostatic pressure linear guide device for machining large-sized work-pieces which requires higher accuracy.
The stiffness and the fluid-utility also depend on the numbers of regulators and shut-off valves provided with fluid-supply pipes in the body of the slide unit. Small numbers of these valves and pipes in the body is better to increase stiffness.
U.S. Pat. No. 6,276,491 and U.S. Pat. No. 6,220,281 show a regulator for controlling oil flow fed from oil supply to the plural number of hydrostatic pockets of the slide unit. A gap between the static pressure guide surface and the slide unit surface to be supported of the hydrostatic bearing is maintained always constant in accordance with the control of the operation oil temperature and the operation oil pressure.
The regulator has a controller housing with a mating surface and has pressure chambers which are separated by a control element, movable against the force of a spring element, and of which a first pressure chamber, subjected to a first pressure level, is directly connected to the medium supply. A second pressure chamber, subjected to a lower, second pressure, is connected to the medium supply via a first flow resistance, and a third pressure chamber, which is subjected to a third pressure level which corresponds to that in the hydrostatic pocket, is connected to the hydrostatic pocket. The control element forming a variable second flow resistance which is arranged between the second and third pressure chambers and influences the medium flow fed to the pocket. The magnitude of the variable second flow resistance being raised by the action of the force of the first pressure chamber but is lowered by the action of the spring force and by the actions of the forces of the second and third pressure chambers. The mating surface is assigned to the slide unit, and the controller housing has a bearing surface, including a passage orifice for the medium flow, with the aid of which in the mounted state the controller housing bears against the mating surface assigned to the slide unit, thus creating a direct medium connection between the controller and pocket and, if appropriate, between the controller and the oil supply.
The regulator is available from Schonfeld GmbH in Germany as a “PM-Flow Controller” (trade name). The regulator can change a quantity of fluid flow variably by valves to control a hydrostatic pressure flexibly and keep a change of the fluid membrane-thickness against the change of the load on the slide unit to a minimum, so that the gaps of the guiding surfaces throughout the machining plane are kept constant. It reduces a fluid demand and increases a stiffness of the hydrostatic pressure linear guide device.